1. Field of the Invention
This application relates generally to the field of polarization controllers for fiber optic applications.
2. Description of the Related Art
In hollow-core photonic-bandgap fibers (PBFs), the majority of the fundamental mode power propagates in air (see, e.g., specifications for HC-1550-02 hollow-core photonic-bandgap fiber available from Crystal Fibre A/S of Birkerød, Denmark). This property makes hollow-core fibers promising for a number of applications, including those in which high peak powers and/or low nonlinearity are desired.
In general, it is desirable to be able to control the state of polarization (SOP) of light propagating in a fiber, and currently no such means exist in hollow-core fibers. In conventional single-mode fibers (SMFs), polarization control is routinely achieved by bending the fiber into loops to induce birefringence through strain (see, e.g., H. C. Lefevre, “Single mode fractional wave devices and polarisation controllers,” Electronics Letters, Vol. 16, pages 778-780 (1980)). FIG. 1 schematically illustrates an SMF bent to form a pair of loops having a radius of curvature R. The induced birefringence Δn is inversely proportional to the square of the radius of curvature (i.e., Δn ∝ 1/R2). The total phase delay δl produced by the loops is proportional to the number of loops Nloops divided by the radius of curvature of the loops (i.e., δl ∝ Nloops/R). For an SMF28 fiber, a quarter-wave plate can be produced using two loops each having a radius of curvature of about 2.5 centimeters. Two such quarter-wave plates can be used as a true universal polarization controller to transform any input state of polarization (SOP) into any output SOP. Such a polarization controller can be described as transforming any input SOP to an output SOP that reaches any point on the Poincaré sphere by both rotating the SOP and changing its ellipticity.